Aberration mark and method for estimating overlay error and optical aberrations

ABSTRACT

An aberration mark for use in an optical photolithography system, and a method for estimating overlay errors and optical aberrations. The aberration mark includes an inner polygon pattern and an outer polygon pattern, wherein each of the inner and outer polygon patterns include a center, and two sets of lines and spaces having a different feature size and pitch that surround the outer polygon pattern. The aberration mark can be used to estimate overlay errors and optical aberrations. In some embodiments, the mark can also be used with scatterometry or scanning electron microscope devices. In other embodiments, the mark can be used to monitor aberrations of a lens in an optical photolithography system.

FIELD OF THE INVENTION

The present invention is related to optical photolithography, and moreparticularly to the measurement of overlay errors and opticalaberrations.

BACKGROUND INFORMATION

The manufacture and fabrication of semiconductor devices involve complexprocessing steps. During the manufacture of integrated circuits, manylayers of different materials are applied to a substrate. These layersoverlie one another and must be accurately registered to ensure properoperation of the semiconductor device. If the layers are not properlyaligned, the device may not perform well, or may even be inoperative. Assemiconductor devices have increased in complexity, the featuredimensions of these devices have decreased, and the influences ofoptical aberrations become more significant.

To aid in the registration of overlying layers in semiconductor devices,registration patterns, or marks, are included in each layer of the waferused during fabrication. These patterns have a predeterminedrelationship when they are correctly registered. A reticle is used topattern the appropriate marks on a particular wafer process layer, suchthat the marks can be readily identified by a Registration tool insubsequent processing steps. One example of an alignment mark is abox-in-box mark. An outer box is formed by photolithography, and aninner smaller box is formed in a separate photolithography layeringstep. When the two boxes are concentric, the layers are accuratelyregistered. Any alignment error produces a displacement of the boxesrelative to each other.

Because semiconductor devices are complex and expensive to fabricate, itis desirable to verify registration after the application of each layer.If the displacement of layers is outside of the acceptable limits,defective layers can then be removed and replaced. Registrationmeasurement, verification, and correction is therefore critical to thesuccessful fabrication of these semiconductor devices.

Registration measurement, verification, and correction can be limited byoptical aberrations introduced during the photolithography process.Aberration errors are of particular significance given the reduction ofsizes of patterns in semiconductor devices. Aberrations affect theability to accurately measure overlay error. Shift quantity measurementsmay not correspond to the actual shift quantities.

There are different forms of aberrations that can affect registrationverification. Coma aberration exerts the largest influence on thedetermination of overlay error. Shift of a wave front caused by comaaberration is large at a peripheral portion of a lens and is small at acentral portion. Diffracted rays of a large semiconductor pattern arepot significantly affected by coma aberration because they have a smalldiffraction angle and pass through a central region of a lens, causingless wave front aberration. However, a small semiconductor patternallows passage higher frequency light, which will be more affected by adiffraction phenomenon of a lens. Therefore, the rays diffracted by asmall semiconductor pattern have a large diffraction angle, and passthrough a peripheral region of a lens, thereby exhibiting more of a comaaberration.

Astigmatism is another optical aberration that occurs because a wavesurface in general has double curvature. In this form of aberration, therays from an object point do not come to a point focus, but ratherintersect a set of image planes in a set of ellipses, the diameters ofwhich are proportional to the distances of the two foci from the imageplane in consideration.

Spherical aberrations have symmetry of rotation, and aredirection-independent. These aberrations occur because rays of differentaperture usually do not come to the same focus. These aberrations arealso sometimes referred to as aperture aberrations. Spherical aberrationoccurs in simple refraction at a spherical surface, and is characterizedby peripheral and paraxial rays focusing at different points along theaxis.

As discussed earlier, semiconductor devices have increased incomplexity. The feature dimensions of these devices have decreased, andthe influences of overlay errors and optical aberrations have becomemore significant. It is critical that both overlay errors and opticalaberrations be estimated accurately and easily to optimize the criticaldimension manufacturing process.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need for analignment mark that can be used to estimate both overlay errors and alsooptical aberrations such as astigmatism, coma, spherical aberration, anddefocus.

SUMMARY OF THE INVENTION

One aspect of the present invention provides an aberration mark for usein an optical photolithography system, and a method for estimatingoverlay errors and optical aberrations. The aberration mark includes aninner polygon pattern and an outer polygon pattern, wherein each of theinner and outer polygon patterns include a center, and two sets of linesand spaces having a different feature size and pitch that surround theouter polygon pattern. The aberration mark can be used to estimateoverlay errors and optical aberrations.

In some embodiments, the inner polygon pattern is a smaller square boxshape and the outer polygon pattern is a larger square box shape. Inother embodiments, the inner polygon pattern is a smaller octagon shapeand the outer polygon pattern is a larger octagon shape.

Another aspect of the present invention provides a method of using anaberration mark during a scatterometry process to estimate opticalaberrations, wherein the aberration mark has two schnitzel patterns ofdifferent pitch. The method includes shining a laser on the aberrationmark at an angle, capturing an image of a scattering of the laser fromthe two schnitzel patterns, measuring a width of the two schnitzelpatterns, estimating a defocus aberration, estimating a coma aberration,estimating a spherical aberration, and estimating an astigmatismaberration.

Yet another aspect of the present invention provides a method of using amark with a scanning electron microscope to estimate overlay errors andoptical aberrations, wherein the mark has a box-in-box structure and twoschnitzel patterns of different pitch that surround the box-in-boxstructure. The method includes scanning the mark with an electron beamin a vacuum, capturing an image of ejected electrons from the twoschnitzel patterns, measuring a width of the two schnitzel patterns,estimating a displacement of the box-in-box structure, estimating adefocus aberration, estimating a coma aberration, estimating a sphericalaberration, and estimating an astigmatism aberration.

Still another aspect of the present invention provides a method formonitoring aberrations of a lens in an optical photolithography system.The method includes forming a reticle on a first mask, the reticlehaving a box-in-box structure and two schnitzel patterns of differentpitch that surround the box-in-box structure, forming a first imagepattern from the reticle during a first photolithography cycle, thefirst image pattern having a box-in-box structure and two schnitzelpatterns of different pitch that surround the box-in-box structure ofthe first image pattern, measuring a first line-shortening effect in thetwo schnitzel patterns of the first image pattern, estimating a baselineset of optical-aberration values of the lens, forming the reticle on asecond mask, forming a second image pattern from the reticle during asecond photolithography cycle, the second image pattern having abox-in-box structure and two schnitzel patterns of different pitch thatsurround the box-in-box structure of the second image pattern, measuringa second line-shortening effect in the two schnitzel patterns of thesecond image pattern, estimating a subsequent set of optical-aberrationvalues of the lens, and comparing the baseline and subsequent set ofoptical-aberration values of the lens to determine changes.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings, where the same number reflects similarfunction in each of the drawings,

FIG. 1 is a high-level view of various components in an opticalphotolithography system;

FIG. 2 is a top view of one embodiment of the present invention thatillustrates an aberration mark according to one embodiment of thepresent invention;

FIG. 3 is an expanded view of one array of parallel lines in theaberration mark that exhibits a line-shortening effect during thephotolithography process;

FIG. 4 is a top view of an aberration mark that is formed on a waferduring the photolithography process according to one embodiment of thepresent invention;

FIG. 5 is an expanded view of a displacement in a box-in-box componentof an embodiment of the aberration mark;

FIG. 6 is a side view of an embodiment of the aberration mark shown inFIG. 4;

FIG. 7 is a top view of another embodiment of the present invention thatillustrates an aberration mark in the shape of an octagon;

FIG. 8 is a perspective view of a wafer and die pattern that includes anembodiment of the aberration mark; and

FIG. 9 is a block-diagram view of a computer system implementing anembodiment of the aberration mark.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the claims.

FIG. 1 is a high-level view of various components in an opticalphotolithography system used with the present invention. The opticalphotolithography system shown is just one example of an environment inwhich the present invention may be practiced. System 100 includes alight source 110, mask 120, optical projection element 130, wafer 150,and wafer registration element 140. Beams of light are emitted fromlight source 110 and pass through mask 120. Mask 120 can include areticle. A reticle has only a portion of a complete die pattern. Opticalprojection element 130 projects and patterns the image from mask 120onto wafer 150. Wafer registration element 140 verifies the fabricationprocess and pattern alignment. System 100 patterns wafer 150 from mask120 and registers wafer 150.

Transferring an image from mask 120 to wafer 150 is a multi-stepprocess. This multi-step process first includes applying photoresistonto wafer 150. Photoresist is a light-sensitive material, such thatexposure to light causes changes in its structure and properties. Lightsource 110, mask 120, optical projection element 130, and wafer 150 mustall be precisely aligned, such that emitted rays from light source 110pass through mask 120. Optical projection element 130 includes one ormore lenses that project the rays through mask 120 onto portions of thephotoresist layer of wafer 150. Negative resist is polymerized, andthose portions of unpolymerized resist are removed. Wafer registrationelement 140 verifies alignment of multiple patterned layers on wafer150. Wafer registration element 140 can include a stepper component.After registration, the top layer of wafer 150 is removed, or etched,through an opening in the resist layer, and the rest of the photoresistlayer is also removed from the wafer. Wafer 150 has now been patternedwith the image from mask 120.

Mask 120 may include a reticle that has registration verification marksthat are used and recognized by wafer registration element 140. However,the accuracy the registration process can be limited by opticalaberrations introduced by optical projection element 130 that affect thepatterning of wafer 150. Optical projection element 130 includes one ormore lenses. The one or more lenses of optical projection element 130can introduce aberrations into the fabrication process.

FIG. 2 is a top view of one embodiment of the present invention thatillustrates an aberration mark according to one embodiment of thepresent invention. This mark is used both for registration and forestimating aberrations introduced by optical projection element 130, andcan be patterned onto wafer 150 from mask 120. Mark 200 includes innerbox 210 and outer box 220. Outer box 220 is larger than inner box 210.Outer box 220 has first, second, third, and fourth outer sides. Mark 200also includes a first and second schnitzel pattern. (The term schnitzelis used in this application to mean an array of lines. A schnitzelpattern can include one or more schnitzels, i.e. arrays of lines.) Firstschnitzel pattern includes first array 230, second array 240, thirdarray 250, and fourth array 260. First array 230 is separate from andadjacent to the first outer side of outer box 220. First array 230includes parallel lines of first pitch P1 that run in a directionperpendicular to the first outer side of outer box 220. Each of thelines of first array 230 has a first length L1. Second array 240 isseparate from and adjacent to the second outer side of outer box 220.Second array 240 includes parallel lines of first pitch P1 that run in adirection perpendicular to the second outer side of outer box 220. Eachof the lines of second array 240 has a first length L1. Third array 250is separate from and adjacent to the third outer side of outer box 220.Third array 250 includes parallel lines of first pitch P1 that run in adirection perpendicular to the third outer side of outer box 220. Eachof the lines of third array 250 has a first length L1. Fourth array 260is separate from and adjacent to the fourth outer side of outer box 220.Fourth array 260 includes parallel lines of first pitch P1 that run in adirection perpendicular to the fourth outer side of outer box 220. Eachof the lines of fourth array 260 has a first length L1.

Second schnitzel pattern includes first array 270, second array 280,third array 290, and fourth array 300. First array 270 is separate fromand adjacent to first array 230. First array 270 includes parallel linesof second pitch P2 that run in a direction perpendicular to the firstouter side of outer box 220. Each of the lines of first array 270 has asecond length L2. Second array 280 is separate from and adjacent tosecond array 240. Second array 280 includes parallel lines of secondpitch P2 that run in a direction perpendicular to the second outer sideof outer box 220. Each of the lines of second array 280 has a secondlength L2. Third array 290 is separate from and adjacent to third array250. Third array 290 includes parallel lines of second pitch P2 that runin a direction perpendicular to the third outer side of outer box 220.Each of the lines of third array 290 has a second length L2. Fourtharray 300 is separate from and adjacent to fourth array 260. Fourtharray 300 includes parallel lines of second pitch P2 that run in adirection perpendicular to the fourth outer side of outer box 220. Eachof the lines of fourth array 300 has a second length L2.

In one specific embodiment, the first pitch P1 of each of the arrays230, 240, 250, and 260 of the first schnitzel pattern is equal to 0.15microns. The first length L1 of each of the parallel lines in thesearrays is equal to 0.15 microns. The second pitch P2 of each of thearrays 270, 280, 290, and 300 of the second schnitzel pattern is equalto 0.5 microns, and the second length L2 of each of the parallel linesin these arrays is equal to 0.25 microns.

In another specific embodiment, the first length L1 of each of theparallel lines in arrays 230, 240, 250, and 260 of the first schnitzelpattern is equal to an amount in the range of 0.15 to 0.35 microns.Second length L2 of each of the parallel lines in arrays 270, 280, 290,and 300 of the second schnitzel pattern is equal to an amount in therange of 0.15 to 0.35 microns.

FIG. 3 is an expanded view of one array of parallel lines in theaberration mark that exhibits a line-shortening effect during thesemiconductor fabrication process. As an example, third array 250 of thefirst schnitzel pattern of mark 200 is shown in FIG. 3. Third array 250has parallel lines each of first length L1. Third array 250 may be on areticle on mask 120 that is to be patterned onto wafer 150 throughoptical projection element 130 (shown in FIG. 1). Third array 250 may bepatterned onto wafer 150 as third (patterned) array 450 having parallellines each of length L1′. However, L1 may not be equal to L1′. In fact,in many instances, L1′ will be less than L1 due to an effect calledline-end shortening. This can also be characterized as a change in thewidth of third array 250. Line-end shortening effects are caused byoptical diffraction during the photolithography process. Opticalprojection element 130 may cause line-end shortening for the patterningof third (patterned) array 450 onto wafer 150. FIG. 3 only shows thisline-shortening effect for third array 250 of mark 200, but this sameeffect may also cause line-end shortening in first array 230, secondarray 240, and fourth array 260 of the first schnitzel pattern of mark200, as well as in first array 270, second array 280, third array 290,and fourth array 300 of the second schnitzel pattern of mark 200. Thisline-end shortening will have a potential effect on all of the imagedpatterns of these arrays on wafer 150. Measurements of the line-endshortening in the patterned images of the arrays of the first and secondschnitzel patterns with respect to the inner and outer boxes allows oneto calculate estimates of aberrations. Estimations of aberrations withthe present invention can be either exact or approximate valuesdepending on the nature of the measurements of the line-end shorteningin the patterned images of the arrays of the first and second schnitzelpatterns.

FIG. 4 is a top view of an aberration mark that is formed on a waferduring the semiconductor fabrication process according to one embodimentof the present invention. The aberration mark 200 is patterned onto awafer to form pattern 400, as shown in FIG. 4. Pattern 400 includesinner box 410 and outer box 420. Outer box 420 is larger than inner box410. Outer box 420 has first, second, third, and fourth outer sides.Pattern 400 also includes a first and second schnitzel pattern. Firstschnitzel pattern includes first array 430, second array 440, thirdarray 450, and fourth array 460. First array 430 is separate from andadjacent to the first outer side of outer box 420. First array 430includes parallel lines that run in a direction perpendicular to thefirst outer side of outer box 420. Each of the lines of first array 430may have been subject to a line-shortening effect when patterned duringphotolithograph. Second array 440 is separate from and adjacent to thesecond outer side of outer box 420. Second array 440 includes parallellines that run in a direction perpendicular to the second outer side ofouter box 420. Each of the lines of second array 440 may have beensubject to a line-shortening effect when patterned duringphotolithograph. Third array 450 is separate from and adjacent to thethird outer side of outer box 420. Third array 450 includes parallellines that run in a direction perpendicular to the third outer side ofouter box 420. Each of the lines of third array 450 may have beensubject to a line-shortening effect when patterned duringphotolithograph. Fourth array 460 is separate from and adjacent to thefourth outer side of outer box 420. Fourth array 460 includes parallellines that run in a direction perpendicular to the fourth outer side ofouter box 420. Each of the lines of fourth array 460 may have beensubject to a line-shortening effect when patterned duringphotolithograph.

Second schnitzel pattern includes first array 470, second array 480,third array 490, and fourth array 500. First array 470 is separate fromand adjacent to first array 430. First array 470 includes parallel linesthat run in a direction perpendicular to the first outer side of outerbox 420. Each of the lines of first array 470 may have been subject to aline-shortening effect when patterned during photolithograph. Secondarray 480 is separate from and adjacent to second array 440. Secondarray 480 includes parallel lines that run in a direction perpendicularto the second outer side of outer box 420. Each of the lines of secondarray 480 may have been subject to a line-shortening effect whenpatterned during photolithograph. Third array 490 is separate from andadjacent to third array 450. Third array 490 includes parallel linesthat run in a direction perpendicular to the third outer side of outerbox 420. Each of the lines of third array 490 may have been subject to aline-shortening effect when patterned during photolithograph. Fourtharray 500 is separate from and adjacent to fourth array 460. Fourtharray 500 includes parallel lines that run in a direction perpendicularto the fourth outer side of outer box 420. Each of the lines of fourtharray 500 may have been subject to a line-shortening effect whenpatterned during photolithograph.

As discussed above, optical aberrations can cause fabrication errorswhen patterning wafers from the masks during photolithograph. Many ofthese aberrations are size, critical dimension (CD), and pitchdependent. That is, the aberrations will have more or less of aninfluence depending on the size, CD, and line pitch for the images thatare patterned on the wafers. In particular, the line-shortening effectsdiscussed above may have a noticeable influence on the arrays ofparallel lines in any of the embodiments of the aberration mark. Thelines often will not be patterned with the same length as they have inthe mask, and the line-shortening effect can be measured with respect tothe outer box 420 of the patterned image. Measuring such effects willallow calculation of estimates of various aberrations in the lens(es)used during photolithograph.

First array 430 of the first schnitzel pattern is adjacent to the firstouter side (right outer side) of outer box 420. The lines of first array430 may have been shortened due to a line-shortening effect when theywere patterned onto the wafer. These lines run in a directionperpendicular to the right outer side of outer box 420, and a horizontaldistance H3 can be measured between the right outer side of outer box420 and a line-end of the lines in first array 430. Horizontal distanceH3 is shown in FIG. 4. Second array 440 of the first schnitzel patternis adjacent to the second outer side (lower outer side) of outer box420. The lines of second array 440 may have been shortened due to aline-shortening effect when they were patterned onto the wafer. Theselines run in a direction perpendicular to the lower outer side of outerbox 420, and a vertical distance V3 can be measured between the lowerouter side of outer box 420 and a line-end of the lines in second array440. Third array 450 of the first schnitzel pattern is adjacent to thethird outer side (left outer side) of outer box 420. The lines of thirdarray 450 may have been shortened due to a line-shortening effect whenthey were patterned onto the wafer. These lines run in a directionperpendicular to the left outer side of outer box 420, and a horizontaldistance H1 can be measured between the left outer side of outer box 420and a line-end of the lines in third array 450. Fourth array 460 of thefirst schnitzel pattern is adjacent to the fourth outer side (upperouter side) of outer box 420. The lines of fourth array 460 may havebeen shortened due to a line-shortening effect when they were patternedonto the wafer. These lines run in a direction perpendicular to theupper outer side of outer box 420, and a vertical distance V1 can bemeasured between the upper outer side of outer box 420 and a line-end ofthe lines in fourth array 460.

First array 470 of the second schnitzel pattern is adjacent to firstarray 430. The lines of first array 470 may have been shortened due to aline-shortening effect when they were patterned onto the wafer. Theselines run in a direction perpendicular to the right outer side of outerbox 420, and a horizontal distance H4 can be measured between the rightouter side of outer box 420 and a line-end of the lines in first array470. Horizontal distance H4 is shown in FIG. 4. Second array 480 of thesecond schnitzel pattern is adjacent to second array 440. The lines ofsecond array 480 may have been shortened due to a line-shortening effectwhen they were patterned onto the wafer. These lines run in a directionperpendicular to the lower outer side of outer box 420, and a verticaldistance V4 can be measured between the lower outer side of outer box420 and a line-end of the lines in second array 480. Third array 490 ofthe second schnitzel pattern is adjacent to third array 450. The linesof third array 490 may have been shortened due to a line-shorteningeffect when they were patterned onto the wafer. These lines run in adirection perpendicular to the left outer side of outer box 420, and ahorizontal distance H2 can be measured between the left outer side ofouter box 420 and a line-end of the lines in third array 490. Fourtharray 500 of the second schnitzel pattern is adjacent to fourth array460. The lines of fourth array 500 may have been shortened due to aline-shortening effect when they were patterned onto the wafer. Theselines run in a direction perpendicular to the upper outer side of outerbox 420, and a vertical distance V2 can be measured between the upperouter side of outer box 420 and a line-end of the lines in fourth array500.

After all of these horizontal distances H1, H2, H3, and H4, and verticaldistances V1, V2, V3, and V4 have been measured, the line-shorteningeffects can be assessed in both the horizontal and vertical directions,and estimations of various aberrations can be calculated. Aberrationssuch as coma and astigmatism are direction-dependent, while aberrationssuch as spherical aberration are direction-independent. Aberrations suchas coma and spherical aberration are pattern-dependent, whereasastigmatism is not. The standard box-in-box structure allowsregistration tools to measure overlay error only. Pattern 400 shown inFIG. 4 allows estimation of both overlay error (described in more detailbelow) and optical aberrations.

To characterize the best focus conditions, a Focus Exposure Matrix (FEM)must be ran for the aberration mark for one embodiment. Here, the markis run through a matrix of different focus and dose settings on thestandard lithography processed to be analyzed. The various measurementsH1, H2, H3, H4, and V1, V2, V3, and V4 are taken and plotted versusfocus for the different doses. The resulting chart from this will showwhat value of H1, H2, H3, H4, or V1, V2, V3, and V4 is expected at aparticular dose/focus setting.

The first optical aberration that can be estimated with pattern 400 isdefocus. Defocus is a measure of the lack of focus on an image through alens. A lens that is not in focus can cause defocus aberration whenpatterning an image during photolithography. The line-shortening is adirect result of defocus discussed above. Defocus can be estimated inboth the vertical and horizontal directions (or from the average of themboth). A first horizontal defocus HF1 can be estimated from the equation(H1+H3)/2. This estimation uses the measurements for H1 and H3 describedabove for the first schnitzel pattern. H3 measures the line-shorteningeffect in first array 430, and H1 measures the line-shortening effect inthird array 450. HF1 measures the defocus in the horizontal directionusing first array 430 and third array 450 of the first schnitzel patternas reference patterns. A second horizontal defocus HF2 can be estimatedfrom the equation (H2+H4)/2. This estimation uses the measurements forH2 and H4 described above for the second schnitzel pattern. H4 measuresthe line-shortening effect in first array 470, and H2 measure theline-shortening effect in third array 490. HF2 measures the defocus inthe horizontal direction using first array 470 and third array 490 ofthe second schnitzel pattern as reference patterns. HF1 and HF2 may notbe equal, because each is measured from a different set of referencepatterns. Estimating both HF1 and HF2 provides a better estimate of thedefocus aberration in the horizontal direction with respect to schnitzelpatterns of different pitch. Also, by comparing HF1 and HF2, through anFEM, one of ordinary skill in the art can determine and estimate a bestfocus of the lens in the horizontal direction, given the line-shorteningeffect of lines of different pitch.

A first vertical defocus VF1 can be estimated from the equation(V1+V3)/2. This estimation uses the measurements for V1 and V3 describedabove for the first schnitzel pattern. V3 measures the line-shorteningeffect in second array 440, and V1 measures the line-shortening effectin fourth array 460. VF1 measures the defocus in the vertical directionusing second array 440 and fourth array 460 of the first schnitzelpattern as reference patterns. A second vertical defocus VF2 can beestimated from the equation (V2+V4)/2. This estimation uses themeasurements for V2 and V4 described above for the second schnitzelpattern. V4 measures the line-shortening effect in second array 480, andV2 measures the line-shortening effect in fourth array 500. VF2 measuresthe defocus in the vertical direction using second array 480 and fourtharray 500 of the second schnitzel pattern as reference patterns. VF1 andVF2 may not be equal, because each is measured from a different set ofreference patterns. Estimating both VF1 and VF2 provides a betterestimate of the defocus aberration in the vertical direction withrespect to schnitzel patterns of different pitch. Also, by comparing VF1and VF2, one of ordinary skill in the art can determine and estimate abest focus of the lens in the vertical direction, given theline-shortening effect of lines of different pitch.

The next optical aberration that can be estimated with pattern 400 isastigmatism. Astigmatism is another optical aberration that occursbecause a wave surface in general has double curvature. In this form ofaberration, the rays from an object point do not come to a point focus,but rather intersect a set of image planes in a set of ellipses, thediameters of which are proportional to the distances of the two focifrom the image plane in consideration. If an object point is a distancefrom the optical axis then the cone of rays from that point will strikethe lens asymmetrically. Rays that are less parallel to the optical axiswill be focused differently from those that are parallel, or almostparallel, to the optical axis.

Astigmatism is an optical aberration that is direction-dependent. It canbe estimated as a difference between the best focus in each of thehorizontal and vertical directions. As noted above, one of ordinaryskill in the art can determine a best focus in the horizontal directionby comparing the values of HF1 and HF2 through an FEM. Also, bycomparing VF1 and VF2, through an FEM, one of ordinary skill in the artcan determine a best focus in the vertical direction. Astigmatism canthen be estimated by determining the difference between the best focusin the horizontal direction and the best focus in the verticaldirection.

The next optical aberration that can be estimated with pattern 400 iscoma. This can be characterized as a pattern-dependent placement shift.Coma aberration exerts the largest influence on the determination ofoverlay error. Shift of a wave front caused by coma aberration is largeat a peripheral portion of a lens and is small at a central portion.Diffracted rays of a large semiconductor pattern are not significantlyaffected by coma aberration because they have a small diffraction angleand pass through a central region of a lens, causing less wave frontaberration. However, a small semiconductor pattern allows passage higherfrequency light, which will be more affected by a diffraction phenomenonof a lens. Therefore, the rays diffracted by a small semiconductorpattern have a large diffraction angle, and pass through a peripheralregion of a lens, thereby exhibiting more of a coma aberration.

Coma is also an aberration that is direction-dependent, and it thereforecan be estimated in both the vertical and horizontal directions. Comaalso can be characterized as a pattern-dependent placement shift, and ittherefore is pattern-dependent. Coma will vary depending on the linepitch of the lines in the first and second schnitzel patterns, andtherefore only this type of structure, having arrays of lines withdifferent pitch, will allow estimation of coma in this way. A firsthorizontal coma HC1 can be estimated as the difference between H1 andH3. This estimation uses the measurements for H1 and H3 described abovefor the first schnitzel pattern. H3 measures the line-shortening effectin first array 430, and H1 measures the line-shortening effect in thirdarray 450. HC1 measures the coma in the horizontal direction using firstarray 430 and third array 450 of the first schnitzel pattern asreference patterns. A second horizontal coma HC2 can be estimated as thedifference between H2 and H4. This estimation uses the measurements forH2 and H4 described above for the second schnitzel pattern. H2 measuresthe line-shortening effect in third array 490, and H4 measures theline-shortening effect in first array 470. HC2 measures the coma in thehorizontal direction using first array 470 and third array 490 of thesecond schnitzel pattern as reference patterns. HC1 and HC2 may not beequal, because each is measured from a different set of referencepatterns. In fact, HC1 and HC2 will be different if the pitch of thelines in the first schnitzel pattern and the lines in the secondschnitzel pattern are different, because coma is pattern-dependent.These distinct coma estimations can only be achieved with this type ofstructure.

A first vertical coma VC1 can be estimated as the difference between V1and V3. This estimation uses the measurements for V1 and V3 describedabove for the first schnitzel pattern. V3 measures the line-shorteningeffect in second array 440, and V1 measures the line-shortening effectin fourth array 460. VC1 measures the coma in the vertical directionusing second array 440 and fourth array 460 of the first schnitzelpattern as reference patterns. A second vertical coma VC2 can beestimated as the difference between V2 and V4. This estimation uses themeasurements for V2 and V4 described above for the second schnitzelpattern. V2 measures the line-shortening effect in fourth array 500, andV4 measures the line-shortening effect in second array 480. VC2 measuresthe coma in the vertical direction using fourth array 500 and secondarray 480 of the second schnitzel pattern as reference patterns. VC1 andVC2 may not be equal, because each is measured from a different set ofreference patterns. In fact, VC1 and VC2 will be different if the pitchof the lines in the first schnitzel pattern and the lines in the secondschnitzel pattern are different, because coma is pattern-dependent.These distinct coma estimations can only be achieved with this type ofstructure. So, as way of an example, if neither HC1 nor HC2 issubstantially equal to zero, then there is coma in the horizontaldirection.

The next optical aberration that can be estimated with pattern 400 isspherical aberration. This can be characterized as a pattern-dependentfocus shift. Spherical aberrations have symmetry of rotation. Theseaberrations occur because rays of different aperture usually do not cometo the same focus. These aberrations are also sometimes referred to asaperture aberrations. Spherical aberration occurs in simple refractionat a spherical surface, and is characterized by peripheral and paraxialrays focusing at different points along the axis. The focal length ofthe lens will vary depending on the distance from the center of thelens. The effect is that a parallel ray of light entering the lens nearthe center will be focused less or more than a parallel ray enteringnear the edges of the lens.

Spherical aberration is direction-independent, and it therefore can beestimated in either the vertical and horizontal directions. Sphericalaberration also can be characterized as a pattern-dependent focus shift,and it therefore is pattern-dependent. It can be estimated as adifference between the focus in either of the horizontal or verticaldirections. Thus; spherical aberration may be first estimated as thedifference between HF1 and HF2. Spherical aberration may also beestimated as the difference between VF1 and VF2. Spherical aberrationmay be used to help estimate the best focus of the lens in thephotolithography system. Because spherical aberration characterizes apattern-dependent focus shift, and because both of either HF1 and HF2 orVF1 and VF2 are required in its estimation, spherical aberration canonly be measured with an embodiment of the aberration mark of thecurrent invention, having arrays of lines in the first schnitzel patternof first pitch P1, and arrays of lines in the second schnitzel patternof second pitch P2. The difference in pitch creates a differentline-shortening effect in the first and second schnitzel patterns, andcreates a pattern-dependent focus shift that can then be estimated.

In some embodiments of the invention, the first, second, third, andfourth arrays of the first schnitzel pattern may not be equally spacedfrom the first, second, third, and fourth outer sides of the outer boxin the mark that is placed on the mask (i.e. there may be inneroffsets). In addition, the first, second, third, and fourth arrays ofthe second schnitzel pattern may not be equally spaced from the first,second, third, and fourth arrays of the first schnitzel pattern in themark (i.e. there may be outer offsets). When the mark is image patternedonto the wafer during fabrication, overlay error and optical aberrationscan be measured in a similar fashion to the method outlined above.However, all distance measurements between the outer sides of the outerbox and the arrays of the first and second schnitzel patterns must bemade relative to the inner and outer offsets noted above.

FIG. 5 is an expanded view of a displacement in a box-in-box componentof an embodiment of the aberration mark. FIG. 5 shows outer box 420 andinner box 410 as a box-in-box structure of pattern 400. FIG. 5 does notshow either the first or second schnitzel patterns of pattern 400. FIG.5 shows an off-center displacement between outer box 420 and inner box410, characterized as a translational-displacement error, in both thehorizontal and vertical directions. In one embodiment of the invention,a displacement between the centers of the outer box and inner box isselected to be zero in the reticle, absent any misalignment induced byprocess fabrication steps. However, process fabrication can introducedisplacement error in either or both of the horizontal and verticaldirections when the image is patterned, such that the center of theouter box is no longer equal to the center of the inner box. Thedisplacement can be measured to estimate overlay error, and can bedetermined with reference to respective, orthogonal, and intersecting Xand Y axes defined as lying in the plane. The intersection of the X andY axes may define an x, y coordinate (0,0), which can be used as areference point in measuring displacement. FIG. 5 shows both ahorizontal and vertical displacement of inner box 410 with respect toouter box 420. The center of inner box 410 has a vertical displacementof Y_(D) with respect to the center of outer box 420, and has ahorizontal displacement of X_(D) with respect to the center of outer box420. These vertical and horizontal displacements are thetranslational-displacement error.

In another embodiment, the displacement between the centers of the outerbox and inner box in a reticle is pre-selected to be of a known non-zeromagnitude and a known direction, absent any misalignment induced byprocess fabrication steps. Any displacement between the centers of theouter box and the inner box in the imaged pattern must be measuredrelative to this pre-selected known offset.

FIG. 6 is a side view of an embodiment of an embodiment of theaberration mark shown in FIG. 4. Pattern 400 is imaged onto a wafer, andthe various layers are shown in FIG. 6. In this embodiment, substrate610 is a base layer of the wafer. First layer 620 overlies substrate 610and is an insulator layer. Second layer 630 overlies first layer 620 andincludes outer box 420, first array 430 of the first schnitzel pattern,first array 470 of the second schnitzel pattern, third array 450 of thefirst schnitzel pattern, and third array 490 of the second schnitzelpattern. Third layer 640 overlies layer 630, and includes inner box 410.FIG. 6 shows only one embodiment of the patterned image of theaberration mark on a wafer. Other embodiments may also implement thisstructure. For example, there may be fewer or additional intermediarylayers. Other embodiments may include inner box 410 being formed on alower layer than outer box 420. FIG. 6 is not intended to limit thestructure of the layers formed on the wafer.

FIG. 7 is a top view of another embodiment of the present invention thatillustrates an aberration mark in the shape of an octagon. Mark 700includes inner octagon 710 and outer octagon 720. Outer octagon 720 hasfirst, second, third, fourth, fifth, sixth, seventh, and eighth outersides. Mark 700 also includes a first and a second schnitzel pattern.First schnitzel pattern includes first array 730, second array 740,third array 750, fourth array 760, fifth array 770, sixth array 780,seventh array 790, and eighth array 800. Second schnitzel patternincludes first array 810, second array 820, third array 830, fourtharray 840, fifth array 850, sixth array 860, seventh array 870, andeighth array 880.

In the first schnitzel pattern, first array 730 is separate from andadjacent to the first outer side of outer octagon 720. First array 730includes parallel lines of first pitch P3 that run in a directionperpendicular to the first outer side of outer octagon 720. Each of thelines of first array 730 has a first length L3. Second array 740 isseparate from and adjacent to the second outer side of outer octagon720. Second array 740 includes parallel lines of first pitch P3 that runin a direction perpendicular to the second outer side of outer octagon720. Each of the lines of second array 740 has a first length L3. Thirdarray 750 is separate from and adjacent to the third outer side of outeroctagon 720. Third array 750 includes parallel lines of first pitch P3that run in a direction perpendicular to the third outer side of outeroctagon 720. Each of the lines of third array 750 has a first length L3.Fourth array 760 is separate from and adjacent to the fourth outer sideof outer octagon 720. Fourth array 760 includes parallel lines of firstpitch P3 that run in a direction perpendicular to the fourth outer sideof outer octagon 720. Each of the lines of fourth array 760 has a firstlength L3. Fifth array 770 is separate from and adjacent to the fifthouter side of outer octagon 720. Fifth array 770 includes parallel linesof first pitch P3 that run in a direction perpendicular to the fifthouter side of outer octagon 720. Each of the lines of fifth array 770has a first length L3. Sixth array 780 is separate from and adjacent tothe sixth outer side of outer octagon 720. Sixth array 780 includesparallel lines of first pitch P3 that run in a direction perpendicularto the sixth outer side of outer octagon 720. Each of the lines of sixtharray 780 has a first length L3. Seventh array 790 is separate from andadjacent to the seventh outer side of outer octagon 720. Seventh array790 includes parallel lines of first pitch P3 that run in a directionperpendicular to the seventh outer side of outer octagon 720. Each ofthe lines of seventh array 790 has a first length L3. Eighth array 800is separate from and adjacent to the eighth outer side of outer octagon720. Eighth array 800 includes parallel lines of first pitch P3 that runin a direction perpendicular to the eighth outer side of outer octagon720. Each of the lines of eighth array 800 has a first length L3.

In the second schnitzel pattern, first array 810 is separate from andadjacent to first array 730 of the first schnitzel pattern. First array810 includes parallel lines of second pitch P4 that run in a directionperpendicular to the first outer side of outer octagon 720. Each of thelines of first array 810 has a second length L4. Second array 820 isseparate from and adjacent to second array 740 of the first schnitzelpattern. Second array 820 includes parallel lines of second pitch P4that run in a direction perpendicular to the second outer side of outeroctagon 720. Each of the lines of second array 820 has a second lengthL4. Third array 830 is separate from and adjacent to third array 750 ofthe first schnitzel pattern. Third array 830 includes parallel lines ofsecond pitch P4 that run in a direction perpendicular to the third outerside of outer octagon 720. Each of the lines of third array 830 has asecond length L4. Fourth array 840 is separate from and adjacent tofourth array 760 of the first schnitzel pattern. Fourth array 840includes parallel lines of second pitch P4 that run in a directionperpendicular to the fourth outer side of outer octagon 720. Each of thelines of fourth array 840 has a second length L4. Fifth array 850 isseparate from and adjacent to fifth array 770 of the first schnitzelpattern. Fifth array 850 includes parallel lines of second pitch P4 thatrun in a direction perpendicular to the fifth outer side of outeroctagon 720. Each of the lines of fifth array 850 has a second lengthL4. Sixth array 860 is separate from and adjacent to sixth array 780 ofthe first schnitzel pattern. Sixth array 860 includes parallel lines ofsecond pitch P4 that run in a direction perpendicular to the sixth outerside of outer octagon 720. Each of the lines of sixth array 860 has asecond length L4. Seventh array 870 is separate from and adjacent toseventh array 790 of the first schnitzel pattern. Seventh array 870includes parallel lines of second pitch P4 that run in a directionperpendicular to the seventh outer side of outer octagon 720. Each ofthe lines of seventh array 870 has a second length L4. Eighth array 880is separate from and adjacent to eighth array 800 of the first schnitzelpattern. Eighth array 880 includes parallel lines of second pitch P4that run in a direction perpendicular to the eighth outer side of outeroctagon 720. Each of the lines of eighth array 880 has a second lengthL4.

Mark 700 has characteristics of an octagon and can also be used topattern an image onto a wafer during photolithography. The imagedpattern can then be used to estimate overlay error and opticalaberrations in a way similar to that outlined above. However, the shapeof outer octagon 720, and the octagon shape of the arrays of the firstand second schnitzel patterns allow one to estimate some of theaberrations in angles of +/−45 degrees, in addition to the horizontaland vertical directions. For example, those aberrations that aredirection-dependent, such as coma and astigmatism, can be measured inthe horizontal and vertical directions, and also in the directions of+/−45 degrees. Measurements like this can be made because the first andsecond schnitzel patterns, and the outer sides of outer octagon 720, runin directions of horizontal, vertical, and +/−45 degrees. Theseadditional measurements would improve the calculated estimations of thedirectionally dependent aberrations.

In other embodiments of the present invention, a mark is a registrationstructure used in calibrating an optical lithographic measurementsystem. The registration structure includes one or more calibrationstructures. Each calibration structure is included on a mask, and can bepatterned onto a wafer during photolithography. Each calibrationstructure includes an inner polygon pattern and an outer polygonpattern. Each of the inner and outer polygon patterns has a center and aplurality of outer sides. Two sets of lines and spaces having adifferent feature size and pitch surround each outer side of the outerpolygon pattern. The displacement between the inner polygon pattern andthe outer polygon pattern is of a pre-selected value. This registrationstructure can be used to estimate both overlay error as well as opticalaberrations. Aberrations that are direction-dependent can be estimatedin the directions of all the angles of the respective sides of the outerpolygon pattern.

In another embodiment, the aberration mark of the present invention isused in the scatterometry process to estimate optical aberrations. Theaberration mark includes two schnitzel patterns of different pitch. Themark is patterned onto a wafer, and scatterometry metrology tests andevaluates the wafer during the fabrication process. Accuracy in opticalsystems is typically limited by the wavelength of the light used, suchthat surface features on the wafer smaller than the wavelength cannot bedetected. However, with scatterometry, a scattered beam can giveinformation about surface features smaller than the wavelength. A laserscans the surface of the wafer, and laser beams are scattered from thesurface onto a screen. A camera then captures the screen image andreconstructs the surface. Scatterometry has the potential of measuringgrain sizes, contours, and critical dimensions (CDs). In this embodimentof the invention, a laser scans the wafer at an angle. The wafercontains the patterned aberration mark. The laser beams are scatteredfrom the surface and an image is captured. The width of the twoschnitzel patterns are measured to calculate the line-shorteningeffects. Estimates then can be made of optical aberrations such asdefocus, coma, spherical aberration, and astigmatism.

In yet another embodiment, the aberration mark of the present inventionis used by a scanning electron microscope (SEM) to estimate opticalaberrations. The aberration mark includes a box-in-box structure and twoschnitzel patterns of different pitch that surround the box-in-boxstructure. The mark is patterned onto a wafer, and an SEM tests andevaluates the wafer during the fabrication process. Conventionalmicroscopes are often limited in their ability to provide accurate datawhen evaluating the wafer surface. Their resolving power is limited bythe wavelength of the light source used. Depth of field is anotherlimitation. In a conventional microscope, the depth of field decreasesas the magnification of the system is increased. Magnification inconventional microscopes is another limiting factor. All of theselimitations are overcome by using an SEM to evaluate a wafer. In an SEM,the illumination source is an electron beam that scans over the wafersurface. Secondary electrons are ejected from the surface and arecollected and translated into an image of the surface. Both the waferand the beam of electrons are in a vacuum. In this embodiment of theinvention, an electron beam of the SEM is used to scan the wafer surfacein a vacuum. The wafer contains the patterned mark. An image of ejectedelectrons from the schnitzel patterns on the mark are captured, so thatthe width of the two schnitzel patterns of different pitch can bemeasured to calculate the line-shortening effects. The measurements withthe SEM can then be used to estimate overlay errors and opticalaberrations. A displacement of the box-in-box structure of the patternestimates overlay error. The optical aberrations that can be estimatedinclude defocus, coma, spherical aberration, and astigmatism.

In yet another embodiment of the present invention, aberrations of alens in an optical photolithography system can be monitored over time.In this embodiment, a reticle is formed on a mask. The reticle includesan aberration mark that has a box-in-box structure surrounded by twoschnitzel patterns of different pitch. The reticle on the mask ispatterned onto a wafer during a photolithography process. Theline-shortening effect in the two schnitzel patterns are then measured,so that estimates can be made of optical aberration values of the lensused for photolithography. (In fact, one or more lenses can be usedduring photolithography, but reference will be made only to a singlelens for simplicity). Some of the optical aberrations of the lens thatcan be estimated include defocus, coma, spherical aberration, andastigmatism. These estimates provide a set of baseline estimates, oraberration fingerprints, of the lens. The quality of a lens can changeover time. By monitoring the quality of the lens, one skilled in the artcan best determine if the lens must be modified or replaced during aphotolithography cycle. This can be done by making subsequent aberrationestimates of the lens and comparing them to the set of baselineestimates. The reticle having the aberration mark is formed on anothermask. The reticle on this mask is patterned onto another wafer, suchthat the line-shortening effect of the two schnitzel patterns can bemeasured. Another set of estimates then can be made of the opticalaberrations of the lens. These aberrations again can include defocus,coma, spherical aberration, and astigmatism. These second set ofestimates can be compared to the set of baseline estimates to determineif any of the values have changed. Certain aberrations may have becomemore predominant or noticeable over time. One of skill in the art canmonitor the photolithography lens over time to compare aberrationestimates with the set of baseline estimate and determine if any changesneed to be made to the photolithography process.

FIG. 8 is a perspective view of a wafer and die pattern that includes anaberration mark of the present invention. Wafer 150 includes die pattern151. In one embodiment, wafer 150 includes one or more marks of thepresent invention. In another embodiment, die pattern 151 is dividedinto individual chips, as is well-known in the art. These chips caninclude one or more integrated circuits. An individual chip can alsoinclude the mark of the present invention.

FIG. 9 is a block-diagram view of a computer system implementing anaberration mark of the present invention. In this embodiment, computersystem 1000 contains a processor 1010 and a memory system 1002 housed ina computer unit 1005. The memory system 1002 includes the aberrationmark of the present invention. Processor 1010 may also include the markof the present invention. Computer system 1000 optionally containsuser-interface components. These user interface components include akeyboard 1020, a pointing device 1030, a monitor 1040, a printer 1050,and a bulk storage device 1060. It will be appreciated that othercomponents are often associated with computer system 1000 such asmodems, device driver cards, additional storage devices, etc. It willfurther be appreciated that the processor 1010 and memory system 1002 ofcomputer system 1000 can be incorporated on a single integrated circuit.Such single-package processing units reduce the communication timebetween the processor and the memory system.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. Therefore, it is intended that this invention be limited onlyby the claims and the equivalents thereof.

What is claimed is:
 1. An aberration mark for use in an opticalphotolithography system, the aberration mark comprising: an innerpolygon pattern; an outer polygon pattern; and two sets of lines andspaces having a different feature size and pitch that surround the outerpolygon pattern for use in determining aberration in a photolithographysystem.
 2. The aberration mark of claim 1, wherein the inner polygonpattern is a smaller square box shape and the outer polygon pattern is alarger square box shape.
 3. The aberration mark of claim 1, wherein theinner polygon pattern is a smaller octagon shape and the outer polygonpattern is a larger octagon shape.
 4. A method for estimating overlayerror and optical aberrations in a photolithography system, the methodcomprising: forming an outer polygon pattern on a first layer; forming aplurality of first arrays around the outer polygon pattern, theplurality of first arrays including one or more parallel lines having afirst pitch; forming a plurality of second arrays around the pluralityof first arrays, the plurality of second arrays including one or moreparallel lines having a second pitch; forming an inner polygon patternon a second layer; measuring a translational-displacement between theinner polygon pattern and the outer polygon pattern; measuring aline-shortening effect in the first arrays and the second arrays;estimating a defocus aberration; estimating a coma aberration;estimating a spherical aberration; and estimating an astigmatismaberration.
 5. The method of claim 4, wherein the method is performed inan order recited in claim
 4. 6. A calibration structure for use incalibrating an optical photolithography measurement system, thecalibration structure comprising: a registration structure formed on asubstrate and having an inner polygon pattern and an outer polygonpattern as viewed from a point perpendicular to a plane generallydefined by the substrate, wherein each of the inner and outer polygonpatterns include a center and a plurality of outer sides, wherein twosets of lines and spaces having a different feature size and pitchsurround each outer side of the outer polygon pattern, and wherein adisplacement between the centers of the inner and outer polygon patternsis selected to be zero, absent any misalignment induced by processfabrication steps.
 7. The calibration structure of claim 6, wherein theouter polygon pattern is formed from a first layer on the substrate,wherein an intermediary layer covers the outer polygon pattern, andwherein the inner polygon pattern is formed from a second layer on theintermediary layer.
 8. The calibration structure of claim 6, wherein theouter polygon pattern is formed from a first layer on the substrate, andwherein the inner polygon pattern is formed from a second layer on thesubstrate.
 9. The calibration structure of claim 6, wherein thedisplacement between the centers of the inner and outer polygon patternsis determined with reference to respective, orthogonal, and intersectingX and Y axes defined as lying in the plane.
 10. The calibrationstructure of claim 9, wherein the intersection of the X and Y axesdefine an x, y coordinate (0,0), and wherein the centers of the innerand outer polygon patterns is located at x, y coordinate (0,0) on the Xand Y axes.
 11. The calibration structure of claim 6, wherein the innerpolygon pattern is a smaller square box pattern and the outer polygonpattern is a larger square box pattern.
 12. The calibration structure ofclaim 6, wherein the inner polygon pattern is a smaller octagon patternand the outer polygon pattern is a larger octagon pattern.
 13. Acalibration structure for use in calibrating a semiconductormanufacturing system, the calibration structure comprising: an opticalmeasurement structure formed on a substrate and having an inner polygonpattern and an outer polygon pattern as viewed from a pointperpendicular to a plane generally defined by the substrate, whereineach of the inner and outer polygon patterns include a center and aplurality of outer sides, wherein two sets of lines and spaces having adifferent feature size and pitch surround each outer side of the outerpolygon pattern, and wherein a displacement between the centers of theinner and outer polygon patterns is selected to be of a known non-zeromagnitude and a known direction, absent any misalignment induced byprocess fabrication steps.
 14. The calibration structure of claim 13,wherein the outer polygon pattern is formed from a first layer on thesubstrate, wherein an intermediary layer covers the outer polygonpattern, and wherein the inner polygon pattern is formed from a secondlayer on the intermediary layer.
 15. The calibration structure of claim13, wherein the outer polygon pattern is formed from a first layer onthe substrate, and wherein the inner polygon pattern is formed from asecond layer on the substrate.
 16. The calibration structure of claim13, wherein the displacement between the centers of the inner and outerpolygon patterns is determined with reference to respective, orthogonal,and intersecting X and Y axes defined as lying in the plane.
 17. Thecalibration structure of claim 13, wherein the inner polygon pattern isa smaller square box pattern and the outer polygon pattern is a largersquare box pattern.
 18. The calibration structure of claim 13, whereinthe inner polygon pattern is a smaller octagon pattern and the outerpolygon pattern is a larger octagon pattern.
 19. A registrationstructure for use in calibrating an optical photolithography system, theregistration structure comprising: a first calibration structure formedon a substrate and having a first inner polygon pattern and a firstouter polygon pattern as viewed from a point perpendicular to a planegenerally defined by the substrate, wherein each of the first inner andfirst outer polygon patterns include a center and a plurality of outersides, wherein two sets of lines and spaces having a different featuresize and pitch surround each outer side of the first outer polygonpattern, and wherein a displacement between the centers of the firstinner and first outer polygon patterns is selected to be zero, absentany misalignment induced by process fabrication steps; and one or moresecond calibration structures formed on the substrate and located on thesubstrate in spaced apart relation to each other and to the firstcalibration structure, each of the second calibration structures havinga second inner polygon pattern and a second outer polygon pattern,wherein each of the second inner and second outer polygon patternsinclude a center and a plurality of outer sides, wherein two sets oflines and spaces having a different feature size and pitch surround eachouter side of the second outer polygon pattern, and wherein adisplacement between the centers of the second inner and second outerpolygon patterns is selected to be zero, absent any misalignment inducedby process fabrication steps.
 20. The registration structure of claim19, wherein the first outer polygon pattern is formed from a first layeron the substrate, wherein an intermediary layer covers the first outerpolygon pattern, and wherein the first inner polygon pattern is formedfrom a second layer on the intermediary layer.
 21. The registrationstructure of claim 19, wherein the second outer polygon pattern isformed from a first layer on the substrate, wherein an intermediarylayer covers the second outer polygon pattern, and wherein the secondinner polygon pattern is formed from a second layer on the intermediarylayer.
 22. The registration structure of claim 19, wherein the firstouter polygon pattern is formed from a first layer on the substrate, andwherein the first inner polygon pattern is formed from a second layer onthe substrate.
 23. The registration structure of claim 19, wherein thesecond outer polygon pattern is formed from a first layer on thesubstrate, and wherein the second inner polygon pattern is formed from asecond layer on the substrate.
 24. The registration structure of claim19, wherein the first inner polygon pattern is a smaller square boxpattern and the first outer polygon pattern is a larger square boxpattern.
 25. The registration structure of claim 19, wherein the secondinner polygon pattern is a smaller square box pattern and the secondouter polygon pattern is a larger square box pattern.
 26. Theregistration structure of claim 19, wherein the first inner polygonpattern is a smaller octagon pattern and the first outer polygon patternis a larger octagon pattern.
 27. The registration structure of claim 19,wherein the second inner polygon pattern is a smaller octagon patternand the second outer polygon pattern is a larger octagon pattern.
 28. Amask, comprising: a circuit pattern, the circuit pattern having one ormore reticles, wherein each reticle comprises: an inner box; an outerbox having a plurality of outer sides; a first plurality of arrays ofparallel lines, wherein one of the first plurality of arrays is adjacentto each outer side of the outer box, and wherein each of the firstplurality of arrays has a first pitch; and a second plurality of arraysof parallel lines, wherein one of the second plurality of arrays isadjacent to each of the first plurality of arrays, and wherein each ofthe second plurality of arrays has a second pitch.
 29. A wafer,comprising: one or more patterned reticle images, wherein each patternedreticle image comprises: an inner pattern; an outer pattern having aplurality of outer sides; a first plurality of arrays of parallel lines,wherein one of the first plurality of arrays is adjacent to each outerside of the outer pattern, and wherein each of the first plurality ofarrays has a first pitch; and a second plurality of arrays of parallellines, wherein one of the second plurality of arrays is adjacent to eachof the first plurality of arrays, and wherein each of the secondplurality of arrays has a second pitch.
 30. The wafer of claim 29,wherein the inner pattern is a smaller box and the outer pattern is alarger box.
 31. The wafer of claim 29, wherein the inner pattern is asmaller octagon and the outer pattern is a larger octagon.
 32. A methodfor estimating overlay errors and optical aberrations in a semiconductorfabrication system, the method comprising: exposing a mask to lightbeams emitted from a light source; forming a first registration patternon a first layer of a substrate, the first registration pattern havingan outer box, four interior arrays of parallel lines each having a firstpitch, and four exterior arrays of parallel lines each having a secondpitch, wherein the four interior arrays surround the outer box, andwherein the four exterior arrays surround the four interior arrays;forming a second registration pattern on a second layer of thesubstrate, the second registration pattern having an inner box, suchthat a displacement between a center of the inner box of the secondregistration pattern and a center of the outer box of the firstregistration pattern has a pre-selected value; measuring atranslational-displacement between the inner box and the outer box inboth a horizontal and a vertical direction; estimating a defocusaberration in both the horizontal and vertical directions; estimating acoma aberration in both the horizontal and vertical directions;estimating a spherical aberration in either of the horizontal orvertical directions; and estimating an astigmatism aberration; wherein afirst line-shortening effect of the parallel lines of the four interiorarrays is different than a second line-shortening effect of the parallellines of the four exterior arrays.
 33. The method of claim 32, whereinthe method is performed in an order recited in claim
 32. 34. A method ofusing an aberration mark during a scatterometry process to estimateoptical aberrations, wherein the aberration mark has two schnitzelpatterns of different pitch, and wherein the method comprises: shining alaser on the aberration mark at an angle; capturing an image of ascattering of the laser from the two schnitzel patterns; measuring awidth of the two schnitzel patterns; estimating a defocus aberration;estimating a coma aberration; estimating a spherical aberration; andestimating an astigmatism aberration.
 35. The method of claim 34,wherein the method is performed in an order recited in claim
 34. 36. Amethod of using a mark with a scanning electron microscope to estimateoverlay errors and optical aberrations, wherein the mark has abox-in-box structure and two schnitzel patterns of different pitch thatsurround the box-in-box structure, and wherein the method comprises:scanning the mark with an electron beam in a vacuum; capturing an imageof ejected electrons from the two schnitzel patterns; measuring a widthof the two schnitzel patterns; estimating a displacement of thebox-in-box structure; estimating a defocus aberration; estimating a comaaberration; estimating a spherical aberration; and estimating anastigmatism aberration.
 37. The method of claim 36, wherein the methodis performed in an order recited in claim
 36. 38. An aberrationdetermination mark for determining aberrations when measuring overlayerror between different layers of a wafer, the aberration determinationmark comprising: an outer box formed on a first layer of a substrate,the outer box having a first, second, third, and fourth outer side; afirst schnitzel pattern on the first layer of the substrate, the firstschnitzel pattern comprising: a first array separate from and adjacentto the first outer side of the outer box, the first array havingparallel lines that run in a direction perpendicular to the first outerside of the outer box; a second array separate from and adjacent to thesecond outer side of the outer box, the second array having parallellines that run in a direction perpendicular to the second outer side ofthe outer box; a third array separate from and adjacent to the thirdouter side of the outer box, the third array having parallel lines thatrun in a direction perpendicular to the third outer side of the outerbox; and a fourth array separate from and adjacent to the fourth outerside of the outer box, the fourth array having parallel lines that runin a direction perpendicular to the fourth outer side of the outer box;wherein each of the arrays of the first schnitzel pattern has a firstpitch, and wherein each of the parallel lines in the arrays of the firstschnitzel pattern has a first length; a second schnitzel pattern on thefirst layer of the substrate, the second schnitzel pattern comprising: afirst array separate from and adjacent to the first array of the firstschnitzel pattern, the first array of the second schnitzel patternhaving parallel lines that run in a direction perpendicular to the firstouter side of the outer box; a second array separate from and adjacentto the second array of the first schnitzel pattern, the second array ofthe second schnitzel pattern having parallel lines that run in adirection perpendicular to the second outer side of the outer box; athird array adjacent to the third array of the first schnitzel pattern,the third array of the second schnitzel pattern having parallel linesthat run in a direction perpendicular to the third outer side of theouter box; and a fourth array separate from and adjacent to the fourtharray of the first schnitzel pattern, the fourth array of the secondschnitzel pattern having parallel lines that run in a directionperpendicular to the fourth outer side of the outer box; wherein each ofthe arrays of the second schnitzel pattern has a second pitch, each ofthe parallel lines in the arrays of the second schnitzel pattern has asecond length, and wherein the first schnitzel pattern is in between thesecond schnitzel pattern and the outer box; and an inner box formed on asecond layer of the substrate, wherein the inner box is smaller than theouter box.
 39. The aberration determination mark of claim 38, wherein adisplacement between a center of the outer box and a center of the innerbox is zero, absent any misalignment induced by process fabricationsteps.
 40. The aberration determination mark of claim 38, wherein thefirst pitch of each of the arrays of the first schnitzel pattern isequal to 0.15 microns, the first length of each of the parallel lines inthe arrays of the first schnitzel pattern is equal to 0.15 microns, thesecond pitch of each of the arrays of the second schnitzel pattern isequal to 0.5 microns, and the second length of each of the parallellines in the arrays of the second schnitzel pattern is equal to 0.25microns.
 41. The aberration determination mark of claim 38, wherein thefirst length of each of the parallel lines in the arrays of the firstschnitzel pattern is equal to an amount in the range of 0.15 to 0.35microns.
 42. The aberration determination mark of claim 38, wherein thesecond length of each of the parallel lines in the arrays of the secondschnitzel pattern is equal to an amount in the range of 0.15 to 0.35microns.
 43. A method for estimating a coma optical aberration in anoptical photolithography system, the method comprising: exposing a maskto light beams emitted from a light source; forming an outer box on afirst layer of a substrate, the outer box having left, right, upper, andlower outer sides; forming a first schnitzel pattern around the outerbox on the first layer, the first schnitzel pattern having left, right,upper, and lower arrays of parallel lines of first pitch; forming asecond schnitzel pattern around the first schnitzel pattern on the firstlayer, the second schnitzel pattern having left, right, upper, and lowerarrays of parallel lines of second pitch; forming an inner box on asecond layer of the substrate; measuring a first horizontal distancebetween the left outer side of the outer box and a line-end of the leftarray of parallel lines of the first schnitzel pattern; measuring asecond horizontal distance between the left outer side of the outer boxand a line-end of the left array of parallel lines of the secondschnitzel pattern; measuring a third horizontal distance between theright outer side of the outer box and a line-end of the right array ofparallel lines of the first schnitzel pattern; measuring a fourthhorizontal distance between the right outer side of the outer box and aline-end of the right array of parallel lines of the second schnitzelpattern; measuring a first vertical distance between the upper outerside of the outer box and a line-end of the upper array of parallellines of the first schnitzel pattern; measuring a second verticaldistance between the upper outer side of the outer box and a line-end ofthe upper array of parallel lines of the second schnitzel pattern;measuring a third vertical distance between the lower outer side of theouter box and a line-end of the lower array of parallel lines of thefirst schnitzel pattern; measuring a fourth vertical distance betweenthe lower outer side of the outer box and a line-end of the lower arrayof parallel lines of the second schnitzel pattern; estimating a firsthorizontal pattern-dependent placement shift, the first horizontalpattern-dependent placement shift being substantially equal to thedifference between the first horizontal distance and the thirdhorizontal distance; estimating a second horizontal pattern-dependentplacement shift, the second horizontal pattern-dependent placement shiftbeing substantially equal to the difference between the secondhorizontal distance and the fourth horizontal distance; estimating afirst vertical pattern-dependent placement shift, the first verticalpattern-dependent placement shift being substantially equal to thedifference between the first vertical distance and the third verticaldistance; and estimating a second vertical pattern-dependent placementshift, the second vertical pattern-dependent placement shift beingsubstantially equal to the difference between the second verticaldistance and the fourth vertical distance.
 44. The method of claim 43,wherein the method is performed in an order recited in claim
 43. 45. Amethod for estimating a spherical optical aberration in an opticalphotolithography system, the method comprising: exposing a mask to lightbeams emitted from a light source; forming an outer box on a first layerof a substrate, the outer box having left, right, upper, and lower outersides; forming a first schnitzel pattern around the outer box on thefirst layer, the first schnitzel pattern having left, right, upper, andlower arrays of parallel lines of first pitch; forming a secondschnitzel pattern around the first schnitzel pattern on the first layer,the second schnitzel pattern having left, right, upper, and lower arraysof parallel lines of second pitch; forming an inner box on a secondlayer of the substrate; measuring a first horizontal distance betweenthe left outer side of the outer box and a line-end of the left array ofparallel lines of the first schnitzel pattern; measuring a secondhorizontal distance between the left outer side of the outer box and aline-end of the left array of parallel lines of the second schnitzelpattern; measuring a third horizontal distance between the right outerside of the outer box and a line-end of the right array of parallellines of the first schnitzel pattern; measuring a fourth horizontaldistance between the right outer side of the outer box and a line-end ofthe right array of parallel lines of the second schnitzel pattern;estimating a first horizontal focus, the first horizontal focus beingsubstantially equal to a first average of the first horizontal distanceand the third horizontal distance; estimating a second horizontal focus,the second horizontal focus being substantially equal to a secondaverage of the second horizontal distance and the fourth horizontaldistance; and estimating a pattern-dependent focus shift, the firstpattern-dependent focus shift being substantially equal to thedifference between the first horizontal focus and the second horizontalfocus.
 46. The method of claim 45, wherein the method is performed in anorder recited in claim
 45. 47. A method for estimating a sphericaloptical aberration in an optical photolithography system, the methodcomprising: exposing a mask to light beams emitted from a light source;forming an outer box on a first layer of a substrate, the outer boxhaving left, right, upper, and lower outer sides; forming a firstschnitzel pattern around the outer box on the first layer, the firstschnitzel pattern having left, right, upper, and lower arrays ofparallel lines of first pitch; forming a second schnitzel pattern aroundthe first schnitzel pattern on the first layer, the second schnitzelpattern having left, right, upper, and lower arrays of parallel lines ofsecond pitch; forming an inner box on a second layer of the substrate;measuring a first vertical distance between the upper outer side of theouter box and a line-end of the upper array of parallel lines of thefirst schnitzel pattern; measuring a second vertical distance betweenthe upper outer side of the outer box and a line-end of the upper arrayof parallel lines of the second schnitzel pattern; measuring a thirdvertical distance between the lower outer side of the outer box and aline-end of the lower array of parallel lines of the first schnitzelpattern; measuring a fourth vertical distance between the lower outerside of the outer box and a line-end of the lower array of parallellines of the second schnitzel pattern; estimating a first verticalfocus, the first vertical focus being substantially equal to a firstaverage of the first vertical distance and the third vertical distance;estimating a second vertical focus, the second vertical focus beingsubstantially equal to a second average of the second vertical distanceand the fourth vertical distance; and estimating a pattern-dependentfocus shift, the second pattern-dependent focus shift beingsubstantially equal to the difference between the first vertical focusand the second vertical focus.
 48. The method of claim 47, wherein themethod is performed in an order recited in claim
 47. 49. A method formonitoring aberrations of a lens in an optical photolithography system,the method comprising: forming a reticle on a first mask, the reticlehaving a box-in-box structure and two schnitzel patterns of differentpitch that surround the box-in-box structure; forming a first imagepattern from the reticle during a first photolithography cycle, thefirst image pattern having a box-in-box structure and two schnitzelpatterns of different pitch that surround the box-in-box structure ofthe first image pattern; measuring a first line-shortening effect in thetwo schnitzel patterns of the first image pattern; estimating a baselineset of optical-aberration values of the lens; forming the reticle on asecond mask; forming a second image pattern from the reticle during asecond photolithography cycle, the second image pattern having abox-in-box structure and two schnitzel patterns of different pitch thatsurround the box-in-box structure of the second image pattern; measuringa second line-shortening effect in the two schnitzel patterns of thesecond image pattern; estimating a subsequent set of optical-aberrationvalues of the lens; and comparing the baseline and subsequent set ofoptical-aberration values of the lens to determine changes.
 50. Themethod of claim 49, wherein the method is performed in an order recitedin claim
 49. 51. A method for estimating direction-dependent opticalaberrations in a photolithography system, the method comprising:patterning an inner polygon mark on a substrate, the inner polygon markhaving a plurality of outer sides; patterning an outer polygon mark onthe substrate, the outer polygon mark having a plurality of outer sides;two sets of lines and spaces having a different feature size and pitchthat surround each outer side of the outer polygon mark; estimating acoma aberration in at least one non-horizontal and non-verticaldirection; and estimating an astigmatism aberration in at least onenon-horizontal and non-vertical direction.
 52. The method of claim 51,wherein the method is performed in an order recited in claim
 51. 53. Themethod of claim 51, wherein the patterning of the inner polygon markincludes patterning an inner octagon mark.
 54. The method of claim 51,wherein the patterning of the outer polygon mark includes patterning anouter octagon mark.
 55. The method of claim 51, wherein the estimatingof the defocus aberration includes estimating in angles of +/−45degrees.
 56. The method of claim 51, wherein the estimating of the comaaberration includes estimating in angles of +/−45 degrees.
 57. Themethod of claim 51, wherein the estimating of the astigmatism aberrationincludes estimating in angles of +/−45 degrees.
 58. A die pattern on awafer, the die pattern comprising: one or more chips, wherein at leastone of the one or more chips includes a patterned reticle image, thepatterned reticle image comprising: an inner pattern; an outer patternhaving a plurality of outer sides; a first plurality of arrays ofparallel lines, wherein one of the first plurality of arrays is adjacentto each outer side of the outer pattern, and wherein each of the firstplurality of arrays has a first pitch; and a second plurality of arraysof parallel lines, wherein one of the second plurality of arrays isadjacent to each of the first plurality of arrays, and wherein each ofthe second plurality of arrays has a second pitch.
 59. The die patternof claim 58, wherein the inner pattern is a smaller box and the outerpattern is a larger box.
 60. The die pattern of claim 58, wherein theinner pattern is a smaller octagon and the outer pattern is a largeroctagon.
 61. A computer system comprising: a computer unit, the computerunit having a processor operatively coupled to a memory system, whereinthe memory system comprises: one or more patterned reticle images,wherein each patterned reticle image comprises: an inner pattern; anouter pattern having a plurality of outer sides; a first plurality ofarrays of parallel lines, wherein one of the first plurality of arraysis adjacent to each outer side of the outer pattern, and wherein each ofthe first plurality of arrays has a first pitch; and a second pluralityof arrays of parallel lines, wherein one of the second plurality ofarrays is adjacent to each of the first plurality of arrays, and whereineach of the second plurality of arrays has a second pitch.
 62. Thecomputer system of claim 61, wherein the computer unit further includesa graphics controller operatively coupled to the processor.
 63. Thecomputer system of claim 61, wherein the processor comprises: one ormore patterned reticle images, wherein each patterned reticle imagecomprises: an inner pattern; an outer pattern having a plurality ofouter sides; a first plurality of arrays of parallel lines, wherein oneof the first plurality of arrays is adjacent to each outer side of theouter pattern, and wherein each of the first plurality of arrays has afirst pitch; and a second plurality of arrays of parallel lines, whereinone of the second plurality of arrays is adjacent to each of the firstplurality of arrays, and wherein each of the second plurality of arrayshas a second pitch.
 64. The computer system of claim 61, wherein thecomputer system further includes one or more user-interface componentseach operatively coupled to the computer unit.
 65. The computer systemof claim 64, wherein each of the one or more user-interface componentsis selected from a group consisting of a monitor, a printer, a pointingdevice, a keyboard, and a bulk storage device.